Treatment of magnesium and its alloys



Patented NOY- 29, 1938 a no Drawing. Application March 15, 1937,

a Se'rial-No.130,909

3 Claims. (01. ita -6)",

The invention relates to improvements in out substantially affecting itsoriginal dimensions methods of producing a protective and decorative orweight. The invention then consists of the coating upon articlesfabricated of magnesium method hereinafter fully described andparticuand its alloys. larly pointed out in the claims.

The type of treatment to which magnesium According to our invention,treating solutions 5 l alloy articles have heretofore been subjected tosuitable for the purpose contain as efiective conproduce thereona'conventional protective coatstituents from 1 to 20 per cent of analkali ing generally consists in treating the article-with metal'chromate, and from 0.5 to 10 per cent of an aqueous solution of a saltproducing adchromic sulphate, based upon the dry weight 10 herent waterinsoluble deposit on the metal. Such of the salts, dissolved in water. Agenerally useb 10 coatings, however, have limitations which adfulcomposition is made by dissolving in water versely affect their utility,so that they are sel- 9 per cent of sodium bichromate, Na2Cr2O7 and 1dom used without, a further treatment, as by per cent of chromicsulphate, Cr2(SO4)3. P0- painting. One disadvantage is that they aretassium chromic sulphate (chrome alum) or amseldom uniform inappearance, the shade and monium chromic sulphate may be used, if de- 16depth of color varying, we have found, with the sired, as the source ofchromic sulphate. In carhomogeneity of the metal itself. The variations'rying our invention into effect, the article to be in color and shadeover the surface of a given treated is first cleaned, if n ss in a knowntreated article are particularly marked in cast manner, as by wirebrushing or by pickling in 20 articles, such as die castings. Rolled andformed dilute nitric acid. The cleaned article is then articles alsoexhibit a certain amount of superimmersed in the treating solution,preferably at ficial variation of metal structure and as a the boilingtemperature thereof, for from about. consequence develop non-uniformcoatings when 0.5 to 60 minutes. until the desired coating is subjectedto a conventional chemical treatment. obtained. After the coating hasbeen produced,

This non-uniformity of the chemically produced the article is removedfrom the solution and 25 coatings on fabricated magnesium articles seemsrinsed immediately, preferably in hot water, and to be due primarily tothe different'rates of acthen air dried. I tion of the treating solutionupon various por- The treating solution may contain other intions of themetal surface, and to the fact that gredients and may, after use,contain a minor the protective deposit does not possessa suflicientamount of products of solution of the articles 30 opacity to fully hidethe underlying irregulari treated. It will eventually become exhaustedor ties in the metal surface. Attempts to build up lean in activeingredients and require regenerathicker coatings, to mask theinhomogeneity of tion or renewal. Any solution preparable by distheordinary coatings, do not produce satisfacsolving in water the saidactive ingredients will I Y UNITED PATEN i o rlc-E tory results, becausesuch coatings lack adequate function in accordance with our discovery.35 mechanical strength. Such attempts also involve The following exampleis illustrative of a mode dissolving or etching away an excessive amountof of carrying out the invention:-- the underlying metal, thusmaterially affecting An article composed of a magnesium oy onthedimensional accuracy of the article. taining 0 per cent A1, 0.1 per centMn, 0.5 per 40 a The principal object of the invention is, ac- Bent thece being magnesium, W 0 cordingly, to provide a, method of producing a,mersed f0! 5 minutes at the boiling temperature corrosion-resistantcoating upon magnesium and i a q s treating bath Containing D itsalloys, without excessively attacking or discent of potassium chromicsulphate, K2Cr2(SO4)4 solving the underlying metal, which coating i and9.0 per cent of sodium bichromate, NazCrzOr.

uniform in appearance, irrespective of any nor- The article was thenwithdrawn, rinsed, and 45 mal inhomogeneity of the structure of themetal dried. The treatment produced a uniform, black itself. Otherobjects and advantages will be adherent coating without materialalteration of apparent as the description proceeds. the dimensions orweight of the article.

We have discovered that, by subjecting articles On subjecting the sotreated article to the of magnesium and its alloys to the action of anaction of a 3 per cent solution of sodium chloride 50 aqueous solutioncontaining chromic sulphate by the usual alternate immersion test forresistand an alkali metal chromate or bichromate, a ance to corrosion,it was foundthat the coating uniform adherent corrosion-resistantcoating is retained its original uniform black color for more producedupon the metal, regardless of any superthan two weeks of the test.Similar results are 65 ficial inhomogeneity of the metal itself andwithobtained within the scope of our invention with other magnesiumalloys, the depth of color varying somewhat with the composition of thealloy.

Among the advantages oi the invention are: that'a uniform appearingcoating may be=v produced on articles 01' magnesium and its alloys inspite oi a iackvpt homogeneity in the; metal itself; the coatings arenot only highly decorative, but also possess great resistance tocorrosion.

Other modes of applying the principle of our invention may be employedinstead of the one explained; change being made as regards the methodherein disclosed, provided the steps stated by any of the followingclaims or the equivalent or such stated step-or steps be employed. l I

We therefore particularly point out and distinctly claim asour-invention:

1. In a method of articles of magnesium and its alloys, the stepwhichconsists in subjecting the athlete the step or? a about 1 to'20producing a coating upon action ofan aqueous solution containing as e!-tective constituents chromic sulphate and an alkali metal chromate.

2. In a method of producing a coating upon articles ormagnesium and itsalloys, the step which consists in subjecting the article to the actionof ah aqueous solution containing'as effective constituents from about ito 20 per cent or chromlc sulphate and item about 0.5 to 20 percent'ofan alkali metal chromate. v

In a method of producing a'coating upon articles of magnesium and itsalloys, the step which consists iii-subjecting the article to the actionof an aqueous solution containing from per cent of a chromic sulphate ofthe group consisting of sodium, potassium and boiling temperature.

blchromate at the CHARLES E. NELSON.

HERBERT DE LONG.-

- ammonium and from 0.5 to 20Lper centof sodium

